1. Field of the Invention
The present invention relates to a compact size, high capacitance and high reliability polycrystalline ceramic capacitor.
2. Description of the Prior Art
It has been recently attempted to reduce the thickness of a ceramic dielectric material of a multilayer ceramic capacitor to less than 30 .mu.m and more preferably to less than 20 .mu.m to improve the capacitance per unit volume, reduce the number of electrodes and reduce the size of the capacitor. However, when the thickness of the dielectric material is reduced, the breakdown voltage is lowered accordingly and the lifetime is shortened. Therefore, a highly reliable capacitor could not be attained. In this regard, the particle size of the dielectric material is analyzed below.
In the prior art ceramic capacitor, a material produced by a solid state reaction such as barium titanate which is produced by wet-mixing barium carbonate and titamium oxide, dehydrating and drying the mixture at 1000.degree. C.-1200.degree. C., sintering it and wet-grinding it, is used as the dielectric material. Such a material has a large particle size such as 1-3 .mu.m and the particle size distributes in a wide range. The particle size of the dielectric material of the ceramic capacitor produced by forming the above material under pressure and sintering is much larger than 3 .mu.m and amounts to 6 .mu.m or more and a variation of the particle size is large because of local grain growth of crystal in the sintering process due to impurities and ununiform particle size. When a voltage is applied to such a ceramic capacitor manufactured by the material having the ununiform particle size, an electric field is concentrated on large size particles and the breakdown voltage is lowered, and a distribution curve extends to a low voltage region. Thus, the lifetime is shortened, a probability of break is large and the reliability is low. Accordingly, if such ceramic dielectric material is used in a capacitor with a reduced thickness, the breakdown voltage is further lowered, the lifetime is significantly shortened and many problems will arise.